In high-quality audio encoding, it is well known that different encoding methods are necessary to provide an optimal result with respect to sound quality versus bit rate for a large variety of audio signals. One encoding method may provide good results for certain types of audio signals, whereas other types of audio signals result in poor performance. For very low bit rates, a sinusoidal encoder plus a noise model is most efficient, while waveform encoding techniques generally lead to better results for higher bit rates.
In the current MPEG 2 and MPEG 4 standards, the problem is recognized that different encoding strategies may be more efficient for different bit rates. Thus, a large range of different audio encoders is included in this standard, most of which are targeted to give best results for a limited range of bit rates.
However, normal audio signals include a mix of a large variety of signal properties even within a short period of time. It is therefore quite common that even a few seconds of an audio signal comprise short excerpts dominated by, for example, pure tones, noise, or transients. These different characteristics call for different encoding characteristics for optimal encoding, i.e. the use of a single type of encoder may result in quite poor results in terms of bit rate or quality for certain excerpts of the signal.
Ph.D. work by Scott Levine [1] (see the List of References at the end of the section entitled “description of embodiments”), describes an encoder comprising a mix between a sinusoidal (or parametric) encoder and a waveform encoder. The largest part of an audio signal is encoded with a parametric encoder, while a waveform encoder is used only for the transient parts of the audio signal. In this scheme, a predetermined division between the parametric encoder and the waveform encoder is applied.
U.S. Pat. No. 5,808,569 in the name of Philips describes an encoding scheme in which different parts of a signal are encoded by using two different encoding strategies. However, no further specification is given to determine how bit rate is distributed across the different encoders.
No prior-art audio encoder thus addresses the problem of controlling two or more different encoding schemes in response to varying parameters of an audio signal.